354 



Mr. J. Small. Changes of Electrical 



The scale acted as a light screen, so that the root was not subject to any 

 heliotropic stimulus. 



The Wound Reaction. 



It may be supposed that two wires cannot be inserted into the root-tip 

 without inducing some sort of a wound reaction, and the passage of even an 

 alternating current is almost sure to have some effect on such a delicate 

 structure. The first experiment was, therefore, to determine the resistance 

 at given intervals of time with the root in -the vertical position. Preliminary 

 experiments had shown that the effect of the current could be eliminated by 

 taking the readings within a minute and allowing 15 mins. between each. 

 Fifteen-minute readings, therefore, were taken with the root vertical, and 

 the results show that during the first 30 — 40 mins. there is an injury 

 response which is represented by a decrement of resistance followed by an 

 increment of resistance in two distinct waves, the second being much weaker 

 than the first. The first increment is, in fact, followed by a period of 

 relative quiescence, during which tlie resistance remains almost constant for 

 1 — 2 hours, after which the vitality decreases rapidly, as is shown by the 

 great decrement of the resistance. Yig. 6 is a typical series of readings in 



1 1000 



r 



o' 



time in hours 

 Fig. 6. 



the form of a graph. During the final decrement of resistance the plant can 

 still perceive a change to the horizontal, as is proved by the sudden drop in 

 the graph at —90°. Sometimes if the root was about 2 cm. long it proved 

 very vigorous and the resistance increased for a considerable time (fig. 7) 

 before the loss of vitality began to show. 



The extent of the injury was also tested by growing the beans after they 

 had been used for an experiment. If the readings extended for an hour and 

 a half to two hours, the root afterwards recovered more or less and continued 

 growth with slight abnormalities. If, however, the experiment lasted over 

 2 hours the side of the root- tip in which the wires had been placed decayed 



